1. Field of the Invention
This invention relates to an oxide superconductor comprising Ba as a constituent element and which has excellent superconducting characteristics and a high characteristic stability with time, and also to a method for producing the oxide superconductor.
2. Description of the Prior Art
Oxide superconductor materials having a relatively high superconducting critical temperature, such as LiTi2O3, Ba(Bi, Pb)O3, (Ba, K)BiO3 and the like, have been recently found. Since then, copper oxide superconducting materials having such a high critical temperature, as would never have been expected, have been successfully prepared. Among them, a number of xe2x80x9coxides containing Ba as constituent elementxe2x80x9d have been found as exhibiting a high superconducting critical temperature and a high critical current density.
Such an oxide superconductor containing Ba as a constituent element includes, aside from the ones set out above and found relatively at an initial stage, LnBa2Cu3Oy oxides (wherein Ln stands for one or more of lanthanides) such as YBa2Cu3Oy, Nd(Nd1xe2x88x92xBax)2Cu3O6+xcex4 (xxe2x89xa61) and the like, and oxides of CuBa2Canxe2x88x921CunOy (n=3 or 4), Tl2Ba2Canxe2x88x921CunOy (n=1, 2, 3 or 4), Tl2Ba2Canxe2x88x921CunOy (n=1, 2, 3 or 4), HgBa2Canxe2x88x921CunOy (n=1, 2, 3 or 4) or the like.
All of these oxide superconductors have a high superconducting critical temperature higher than a liquid nitrogen temperature, and their applications and developments as a bulk material, a wire material and a device are now in progress.
For the production of these oxide superconductors, there has been used a procedure which comprises providing, as feed stocks of individual constituent elements, carbonates and oxides which are relatively stable in air {e.g. commercially available Ln2O3 (wherein Ln means one or more of rare earth elements such as La, Nd, Y, Sm and the like), ACO3 (wherein A means Ba, Sr, Ca and the like), CuO and the like}, and weighing these materials at predetermined ratios, followed by mixing, shaping, sintering and cooling (gradual cooling or quenching) to provide an oxide superconductor. It will be noted that after the sintering and cooling, milling, mixing, shaping, sintering and cooling steps have been sometimes repeated several times. Moreover, the resultant oxide is further subjected to the step of thermal treatment (annealing treatment) in an atmosphere of oxygen or the like.
In this case, there have been frequently employed, as the feed stock for Ba, not only BaCO3 mentioned above, but also BaO2 or BaO, or oxides which are obtained by adding, to these oxides, carbonates or oxides of Y, Ca, Cu, Nd and the like and calcining the mixtures (the calcined oxide is hereinafter referred to as xe2x80x9cBa-containing precursorxe2x80x9d).
However, it has been often criticized that such an oxide superconductor containing Ba as the constituent element is poorer in stability over time of superconducting characteristics than Ba-free oxide superconductors {e.g. (La,Sr)2CuOy,Bi2Sr2CaCu2Oy and the like}.
It has also been pointed out that these oxide superconductors containing Ba as the constituent element are more apt to cause, more or less, a lot of variation in ultimate superconducting characteristics than Ba-free oxide superconductors.
These inconveniences become more pronounced with the case of Hg-based oxide superconductors and Tl-based oxide superconductors, and cannot be overlooked in the case of Nd-based oxide superconductors, for which studies on their practical applications have now undergone a marked development.
It is accordingly an object of the invention to provide an oxide superconductor containing Ba as a constituent element, which stably shows good superconducting characteristics without degradation over time.
We made intensive studies in order to achieve the above object and, as a result, found the following.
a) Because BaCO3, BaO or BaO2 is used as a feed stock for the production of an oxide superconductor containing Ba as a constituent element, it is difficult to prevent the incorporation of carbon (C) as an impurity element. More particularly, where BaCO3 is provided as a feed stock for Ba, there is the apprehension of the danger that C will be incorporated therein upon decomposition of BaCO3. On the other hand, when BaO or BaO2 is used as a Ba feed stock, BaO, which is a very unstable compound, is very liable to combine with a carbon-containing gas, such as carbon dioxide or the like, existing in the atmosphere. Moreover, with BaO2, which is usually prepared from BaO as an intermediate material, it is difficult to prevent contamination with a carbon-containing gas through the intermediate material. As a consequence, approximately 1 atomic % (hereinafter referred to simply as % unless otherwise indicated) or more of carbon (C) is incorporated in these starting materials. This eventually permits a substantial amount of carbon (exceeding 2.0%) to be incorporated in an oxide superconductor product through the Ba feed stock along with other contamination during a series of oxide superconductor producing steps.
Especially, where Hg-based oxide superconductors and Tl-based oxide superconductors using high volatilization Hg and Tl, respectively, are produced, heating at a high temperature causes Hg or Tl to evaporate, thereby resulting in crystal lattice defects. In this sense, BaCO3, which is a relatively stable compound and, hence, should be thermally decomposed through a heating treatment at high temperatures to remove the C component, cannot be used. In contrast, BaO, i.e. a compound which is unstable and liable to undergo contamination, has to be used as a Ba feed stock. In the Hg-based or Tl-based oxide superconductors, this makes it more difficult to prevent the incorporation of carbon (C) as an impurity.
In this connection, the carbon incorporated in the oxide superconductor may be partially substituted with a constituent element (e.g. Hg, Tl, Nd or the like) in the crystal lattices of the oxide superconductor to give a strain thereto, or may segregate at the grain boundary when the oxide superconductor is made of a polycrystal. This will cause the degradation of superconducting characteristics and their stability with time of the oxide superconductor.
b) However, when the content of the carbon incorporated, as an impurity, in an oxide superconductor product is reduced to a level of 2.0% or below, the superconducting critical temperature increases for the same type of oxide superconductor along with a marked improvement in the characteristic stability with time.
c) In a hitherto adopted procedure of producing an ordinary oxide superconductor, it has been very difficult to suppress the carbon content in the resulting oxide superconductor product to 2.0% or below. However, when using a conventional procedure wherein there is employed, as a feed stock for Ba, BaO which is prepared under specific conditions and purified satisfactorily so that the content of carbon is reduced to 0.5% or below, the carbon incorporated in the oxide superconductor product as an impurity can be significantly reduced. This enables one to obtain an oxide superconductor product having a carbon content of 2.0% or below, high critical current density and high stability with time.
d) In the case, when the starting materials for oxide superconductor are mixed in a dry atmosphere having a content of a carbon-containing gas, such as carbon dioxide or the like, at 10 ppm or below, preferably 1 ppm or below, and subsequent steps of producing an oxide superconductor are carried out while being isolated from both a carbon-containing gas and moisture, it becomes more preferred for the stable production of a high-performance product.
The invention has been accomplished based on the above findings and provides the following oxide superconductor and a method for producing same.
(1) An oxide superconductor of the type which comprises Ba as a constituent element, characterized in that the content of carbon as an impurity is suppressed to a level of 2.0% or below.
(2) An oxide superconductor of the type which comprises Ba as a constituent element as recited (1) above, wherein the superconductor consists of an oxide of the chemical formula
HgBa2Canxe2x88x921CunOy
wherein n=1, 2, 3 or 4 and y=2n+2+xcex4.
An oxide superconductor of the type which comprises Ba as a constituent element as recited in (1) above, wherein the superconductor consists of an oxide of the chemical formula
Nd(Nd1xe2x88x92xBax)2Cu3O6+xcex4
wherein xxe2x89xa61.
A method for producing an oxide superconductor recited in any one of (1) to (3) above, which method comprising mixing starting materials containing individual constituent elements, shaping, heating and cooling to prepare an oxide superconductor, characterized in that a feed stock for Ba consists of BaO which has a content of a carbon impurity of not greater than 0.5%.
(5) A method for producing an oxide superconductor recited in (4) above, characterized in that all the steps of from the mixing of the feed stocks to the production of the oxide superconductor, are performed in a dry atmosphere whose content of a carbon-containing gas is limited to a level of 10 ppm or below.